The present invention relates to a process for coating metallic substrates which can be used in the fabrication of containers for toxic and hazardous (T & H) wastes, as well as low level radioactive waste (LLRW), and the resulting containers. The coated substrate has enhanced chemical and corrosion resistance, which provides a basis for use under a variety of harsh and/or hostile environments.
In the handling of these types of waste, waste material is often placed in steel barrels with various types of treatments such as encapsulating the waste in chemicals, gelatin, bitumen or cement, burning the waste and placing the burned waste or agglomerate in barrels. The barrels then usually are buried, and on rare occasion have been known to be further encapsulated in a concrete waste-receiving "tomb".
Government regulations regarding the disposal of T & H and LLRW have proliferated during the last decade, and improperly handled waste has generated both the levying of fines and the filing of law suits against the waste disposer.
Waste containers must meet a variety of technical standards because of the large number of materials and types of materials which are disposed of in such containers. In addition, varying environmental conditions are encountered in the disposal of the hazardous waste. Because many waste products are incinerated or processed at elevated temperatures and then immediately placed into a container, the container must also be thermally resistant.
LLRW and T & H wastes and other hazardous materials are, in many cases, presently disposed of by storing in standard, relatively inexpensive, 55 gallon drums, or in high integrity containers (HICs) which are considerably more expensive. Standard 55-gallon steel drums can deteriorate within 20 to 30 years, resulting in leakage or loss of contents. This deterioration is unacceptable for both below ground burial and above-ground storage. Presently used HICs are made of concrete, stainless steel, or polyethylene materials and have several limitations beyond that of high cost. All prior art containers of a 55-gallon drum size have reduced capacity. Each container is thickly lined. Some contain concrete that is 5 or 6 inches thick. Such containers not only have limited capacity, but also are extremely heavy to handle and transport, and use up large amounts of available burial space. Molded polyethylene containers can be biodegradable under certain conditions, they have poor creep performance, and are subject to attack by a variety of chemicals. They also deteriorate under only moderately high temperature, are readily attacked by radiation, and occasionally rupture under pressure. Concrete HICs have extremely high bulk, and they crumble and otherwise deteriorate after only a moderate life span. Stainless steel, on the other hand, must be of a special grade with the consequence of extremely high cost.
Eckner U.S. Pat. No. 4,481,239 teaches a process for coating metallic substrates, particularly pipes, wherein the substrate is preheated. A powder coating is electrostatically applied to the surface of the substrate and melted as a result of the high preheating temperature. The process uses one heat hardenable synthetic resin in combination with hardening agents and cross linkable acrylate resins, a stabilized ethylene copolymer, and optionally polyolefin. These are substantially different materials from those of the present invention, which materials are placed onto a basically steel substrate with the resulting coating substituting for standard pipe wrapping.
Point U.S. Pat. No. 3,513,012 teaches a process for coating an article with substances that are convertible into two continuous but separate solid layers. Electrically charged particles are electrostatically deposited on the first unconverted layer. Point also teaches that the particles are carried to a high electrical potential of about 100 kilovolts as they are discharged adjacent the substrate.
We have invented a method for coating substrates, especially metallic substrates, and in particular for coating a steel drum, which is capable of withstanding high impact, is not subject to corrosion, and thus is extremely well suited for the containment of T & H, LLRW, and other corrosive wastes at relatively low cost. The metallic substrate in the present invention is coated with a combination of a thermosetting resin and a thermoplastic material, which can be simultaneously cured in place.
The invention comprehends high integrity containers, having chemical resistance, radiation resistance, and long term corrosion resistance, and provides a process for making containers which meet tests for such resistance for up to 18,000 to 20,000 hours. Such containers are resistant to attack from corrosive chemicals, caustic chemicals, and radiation.
Until very recently, no coating exceeding around 1000 hours of corrosion resistance to salt spray was available, and until the present invention, no coating exceeded 4000 hours. Applicants' corrosion life of 20,000 hours is far beyond the prior achievements. Such life defines a "high integrity container". Without achieving this result, it would not be possible to extrapolate a 300 year life that the Nuclear Regulatory Commission requires. Standard epoxy with nylon fails to achieve the desired result.
In a high integrity container, when coated in accordance with the invented method, exotic stainless steel can be replaced with inexpensive carbon or alloy steels.
A steel drum, such as a standard 55-gallon drum, is not easily coated by powder coating. Any object having 90 degree corners, and large contained surfaces, develops powder build-ups in the corners. For this reason, drums are not powder-coated today. In addition, in prior techniques, powder is blown off the large surfaces during the coating process, and the coating is not uniform. Drums have been epoxy coated in the past, but not powder coated. The inside of drums have never before been effectively coated.